The present invention relates in one aspect to a single screw extruder for extruding combinations of materials such as thermoplastics polymers, rubbers, waxes and solid additives, and in another aspect to a mixer for such materials. The mixer could however be employed in the manufacture of inks, paints and other materials where one or more of the components is liquid at the room temperature.
Single screw extruders are very widely used in the plastic industry for producing compounds of rubber and thermoplastic polymers with solid additives. They are simple to construct and therefore relatively inexpensive; however that they have limited distributive and dispersive mixing capacity has been long recognised and is well documented (cf xe2x80x9cSingle Screw Mixing: Problems and Solutionsxe2x80x9d Martin Gale, a paper presented at a RAPRA Technology Ltd. seminar Aug. 6, 1995).
Further background as to the mixing limitations of single screw extruders is given in an article in Plastics Additives and Compounding August/September 1995 pages 21-23 entitled xe2x80x9cNew dispersive mixers based on elongational flowxe2x80x9d and the associated patent, U.S. Pat. No. 5,932,159, published Aug. 3, 1999. This stresses the need for a variety of dispersive forces including elongation flow and multiple passes through regions of high stress, conditions which are normally difficult to generate within a single screw machine.
There are many devices that can be used to improve the distributive mixing capacity of single screw extruders; however these devices offer only marginal improvements in dispersive mixing. A good example of this is the cavity transfer mixer (U.S. Pat. No. 4,419,014) where the process melt is transferred repeatedly between cavities in a rotor and opposed cavities in the barrel wall. The rotary motion of the rotor means that the material is constantly subdivided and re-orientated. However this does not generate a high shear rate since the walls of opposing cavities are quite widely separated. In practical devices this low shear rate also limits the maximum cavity size since there is a tendency for the melt to stagnate.
Pins can also be used to improve mixing either protruding radially from the barrel or from the surface of a rotor or the screw itself. Whilst pins do generate chaotic flow, improving distributive mixing, they do little in the way of dispersive mixing since the pins do not move relative to a complementary shear surface. In the case of pinned barrels, gaps in the helical flight sweep over the pins generate high sheer events and also allow significant re-circulation of the polymer melt improving distributive mixing. However the proportion of material subjected to high shear is quite small.
A basic object of the invention is the provision of an improved single screw extruder, and to a mixer for use with such extruder.
According to a first aspect of the invention there is provided a single screw extruder comprising a drivable screw, with at least one flight, located within a static barrel so as to define an annular, material flow gap between the exterior of the screw and the interior of the barrel, a mixer associated with the screw, whereby material passes from an upstream portion of the flow gap, into the mixer and is then either returned to a downstream portion of the flow gap or is discharged, with the mixer comprising a rotor driven by the screw and a stator, the rotor and the stator each carrying mutually facing interengaging rings of teeth whereby the material is urged outwardly from the annular gap along a first tortuous mixing path, and then returned inwardly along a second tortuous mixing path, the teeth extending axially, or generally so, with respect to the longitudinal axis of the screw.
According to a second aspect of the invention there is provided a mixer for mixing solids with liquids, liquids and liquids e.g. polymer alloys, for use with other devices or combinations of devices capable of driving the rotor and introducing material in a fluid state into the mixer under sufficient pressure to cause the material to be mixed to flow through the mixer, the mixer comprising:
(i) a cylindrical stator chamber having opposed, radially extending faces provided with axially projecting, radially spaced-apart rings made up of alternating teeth and ridges,
(ii) a rotor rotatably fitted within the stator and provided on its opposite side faces with axially projecting, radially spaced-apart rings of alternating teeth and ridges, and
(iii) the rotor and stator rings interengaging with both radial and axial clearance so as to define a tortuous material flow path.
The extruder in accordance with the first aspect has been found to provide significant improvement in the extruder performance and the quality of extruded product compared with prior art single screw extruders, whilst the mixer in accordance with the second aspect has been found to be particularly advantageous and to provide a fundamental improvement in the mixing of solids and liquidsxe2x80x94such as a liquid thermoplastics material and solid additive and improve the manufacture of polymer alloys. The rings on the stator may be as thin as possible whilst maintaining mechanical integrity since their only functions are to provide a barrier to melt flow and complementary shear surfaces to the rotor. This arrangement limits possible melt stagnation in the gaps between stator teeth. In addition, the screw may serve as a main bearing for the mixer, whilst because the teeth are concentric around the barrel, there is no constraint on the length of the teeth that may be provided.
The mixer is located intermediate the ends of the screw.
The mixer is located at the discharge end of the screw/extruder.
As the extruder will be used for extruding a range of materials, it is clear that a suite of mixers exhibiting differing geometrical properties to provide different mixing capabilities, is desirable for optimum mixing. Thus, to permit reasonably expedient changing of a mixer, the extruder is provided with readily releasable means eg a pair of releasable flanges, within which the mixer is housed.
The internal diameter of stator chamber is larger than the internal diameter of the extruder to which it is attached and the rotor is relatively short.
Clearly, the mixer can be used in combination with any apparatus capable of introducing the materials to be mixed under sufficient pressure to cause these materials to flow through the mixer. One such apparatus is a single screw extruder. Thus it is necessary to provide the mixer with an entry aperture and an exit aperture for material feed under pressure into, through, and out of, the exit aperture of the mixer. In the mixer the materials undergo four actions namely (i) a radial movement under pressure suitably from a central feed port to an outlet port, (ii) an orbital movement involving division of the radially moving material into portions some of which go one way while vicinal portions go the opposite way and (iii) a shearing action (iv) an elongational deformation.
This mixer differs from that of a conventional extruder configurationxe2x80x94with a long thin screw and any ancillary mixers are contained in a narrow cylinder, i.e. the primary internal barrel diameterxe2x80x94since the mixer has, in it is preferred configurations, a short broad rotor within a chamber with an internal diameter greater than that of the barrel to which it is attached. Looking at this basic geometry two significant advantages become clear. Firstly the shortest path length through the mixer increases only linearly with rotor radius whilst the volume available for mixing increases with the square of that radius. Secondly angular velocity rises linearly with the rotor radius. This means that the largest mixing volume coincides with the highest potential shear rates.
By interengage is meant that the teeth of a rotor or stator ring always extend into the valley defined between a pair of adjacent rings of the stator or rotor, whilst the ridges may or may not extend into that valley. Means are provided to drive angularly the rotor or the stator or both so that there is relative movement between the two. Normally only the rotor will be driven to rotate about its axis.
In one form of the mixer, the rings are concentric to the axis of the rotor. However they may also be arranged eccentrically which results in a cleaning action when the rings approach each other. The maximum eccentricity is limited to the separation between the rings forming the complementary valley on the complementary component.
The maximum combined height of a given ring and tooth at any point on the surface of either the stator or rotor is limited by the separation of the stator and rotor. This separation can vary between 0.1 and 300 mm, preferably 1 to 100 mm.
The combined height of the ridges and teeth can be varied by any amount within this limit either around the circumference or along a radial path. The variation can be either continuous or discontinuous, i.e. the transition can be slopped or stepped, but in the preferred form, the ridges and teeth are uniform in height.
The thickness of both the teeth and ridges around their circumference can be varied but in the preferred form is uniform. This thickness can range from 0.1 mm to 100 mm, preferably 1-30 mm.
Any tooth may combine any or all of these characteristics and the transition between them may be continuous or discontinuous, i.e. the transition may be slopped or stepped.
The stator, rotor, ridges and teeth may be made from any material that is dimensionally stable at the operating temperature and under the mechanical strains generated in operation. Such materials include steel, ceramics, rubber and plastics.
The ridges and teeth can be either permanently or removably attached to the rotor and stator. Removable ridges and teeth may also be so attached as to allow their repositioning and re-orientation. The stator which defines the chamber around the rotor may also be either permanently or releasably assembled around the rotor.
Preferably the stator is defined by a pair of mutually facing cup shaped inserts which are clamped together opening-to-opening. When assembled into a single screw extruder, such clamping would be between the extended flanges of the front and rear barrels. In this way the mixing geometry of the device may be altered by replacing the relatively inexpensive inserts.
In operation, the pressure gradient from inlet of the mixer to outlet causes material to flow through the device. There are three tortuous routes that the material can take. A zigzag route over the intermeshing annular ridges and teeth, a route along the annular channels defined by the ridges and a route through the gaps between teeth. All three routes are continuously changing due to relative motion and take material up one a face of the rotor across its edge and back down the obverse.
In one configuration the gaps between teeth on the stator and the rotor are arranged to form radial channels. The rotational motion of the rotor leads to periodic alignment of teeth and gaps and gaps and gaps between the rotor and stator.
However the gaps may be staggered to alter the mixing characteristics. For any given ring the combined length of teeth and gaps is equal to the circumference of that ridge. The teeth may be of any length along the ring within this total but need not be uniform in length.
Dispersive mixing occurs in the gaps between the faces of the annular ridges and teeth on the rotor and their counterparts on the stator. Material within these gaps is subject to both pressure and drag flow due to the motion of the rotor. In this way the melt in the high shear zone is constantly refreshed. Distributive mixing then ensures that this well-dispersed material is evenly distributed through the bulk of the melt.
Distributive mixing results from the repeated cutting of the melt as it emerges from gaps between teeth on the stator and also as it enters the next set of gaps between the teeth on the stator. Since the speed of flow into and out of these gaps is slower near to the defining teeth than in the middle, significant reorientation of the melt also occurs.
The mixer can be used in conjunction with a number of functional ancillary elements each of which can be supported by a number of well known devices. The ancillary elements include:
a) means of receiving the process material;
b) means of melting one or more of the components to generate liquid;
c) means of degassing the process melt of the inlet side of the mixer;
d) means of generating pressure on the inlet side of the mixer;
e) means of driving the rotor of the mixer;
f) means of degassing the process melt on the outlet side of the mixer;
g) means of generating pressure on the outlet side of the mixer;
h) means of filtering the process melt; and
i) means of forming the process melt. Elements d and e are always required for the operation of the mixer. The need for the other ancillary functional elements is dependent on the users needs, but a single screw extruder can provide functional elements a, b, c, d and e then f, g, h and i. Elements d and e are always required for the operation of the mixer. The need for the other ancillary functional elements is dependent on the users needs. Devices suitable for providing one or more of the functional ancillary elements include:
1) A single screw extruder can provide functional elements a, b, c, d and e then f, g, h and i
2) A twin screw extruder can provide functional elements a, b, c, d and e then f, g, h and i
3) A Z-blade mixer can provide functional elements a, b and c
4) An internal mixer can provide functional elements a, b and c
5) A gear or other pump can provide functional elements d, e and g
6) An external motor can provide functional element e
7) A removable filter system can provide functional element h
8) A removable die can provide functional element i.